Re: [PATCH 2/6] Add returns_zero_on_success/failure attributes

On Wed, Nov 17, 2021 at 10:43:58PM +0000, Joseph Myers wrote:

On Wed, 17 Nov 2021, Prathamesh Kulkarni via Gcc-patches wrote:

quoted

More generally, would it be a good idea to provide attributes for
mod/ref anaylsis ?
So sth like:
void foo(void) __attribute__((modifies(errno)));
which would state that foo modifies errno, but neither reads nor
modifies any other global var.
and
void bar(void) __attribute__((reads(errno)))
which would state that bar only reads errno, and doesn't modify or
read any other global var.

Many math.h functions are const except for possibly setting errno, 
possibly raising floating-point exceptions (which might have other effects 
when using alternate exception handling) and possibly reading the rounding 
mode.  To represent that, it might be useful for such attributes to be 
able to describe state (such as the floating-point environment) that 
doesn't correspond to a C identifier.  (errno tends to be a macro, so 
referring to it as such in an attribute may be awkward as well.)

We need some way to describe these things in Gimple and RTL as well,
and not just on function calls: also on other expressions.  Adding
attributes that allow to describe this (partially, only per function) in
C source code does not bring us closer to where we need to be.


Segher